The present invention relates generally to free machining steels and more particularly to leaded steel bars.
Steel bars may be machined with form tools or drill tools. Typically, a form tool is applied to the peripheral surface of the steel bar and advances inward in a direction normal to the surface, as the bar rotates, whereas a drill tool is applied to the center of the steel bar and advances in an axial direction in a typical operation involving a screw machine.
Form tool machinability may be expressed in terms of cubic inches of metal removed until form tool failure or it may be expressed as part growth, which reflects tool wear which is inversely proportional to machinability. In conventional leaded steel bars, the form tool machinability of the steel bar at the immediate sub-surface region of the bar is significantly lower than the form tool machinability nearer the center or at the interior of the leaded steel bar. The immediate sub-surface region of the steel bar is, for example, at a depth of one-sixteenth - three-sixteenth inch in absolute distance from the surface of the steel bar in a 2-13/16 inches diameter bar. This is about 5-15% of the depth to the center of the bar. As the diameter of the bar decreases from 2-13/16 inches, the immediate sub-surface region in which the lower machinability appears is closer, in absolute distance, to the surface of the bar; and in bars larger than 2-13/16 inches, the immediate sub-surface region lies farther from the surface of the bar, in absolute distance.
In the immediate sub-surface region of conventional leaded steel bars, there are complex macroinclusions whose composition is described below. A macroinclusion is an inclusion which is visible at 1 to 10X magnification. (A microinclusion is an inclusion which is visible at greater than 10X magnification). In rolled bars, the macroinclusions are elongated in the direction of rolling.
As used herein, the term "complex macroinclusions" refers to macroinclusions which comprise oxides of manganese, silicon and iron, with metallic lead interspersed therein. These complex macroinclusions have an adverse effect on machinability. These complex macroinclusions are not the same as the lead macroinclusions disclosed as occurring in the surface or sub-surface region of prior art leaded steel by North et al., U.S. Pat. No. 3,671,224, Col. 2, lines 56-60, which teaches how to avoid such lead macroinclusions and refers to them as lead "streaks or blobs".